1. Technical Field
The present invention relates to an optical element cleaning method, and an optical element cleaning device, for cleaning an optical element having adhered thereon scattered material, generated together with EUV (extreme ultraviolet) light, in an EUV light source device employed as a light source in exposure apparatuses.
2. Related Art
In the wake of ever finer semiconductor processing in recent years, features are quickly becoming finer as well in photolithography. Fine processing requirements are moving from the 100 nm to the 70 nm node, and beyond, to the 50 nm node, in next-generation applications. To meet the requirements of fine processing at the 50 nm node or less, for instance, exposure apparatuses being developed are expected to combine an EUV light source having a wavelength of about 13 nm with a reducing projection reflective optical system (catadioptric system).
There are three types of EUV light sources: LPP (laser produced plasma) light sources, using plasma generated through irradiation of a laser beam onto a target (referred to hereinafter as “LPP-type EUV light source devices”), DPP (discharge produced plasma) light sources, using plasma generated by discharge, and SR (synchrotron radiation) light sources, using synchrotron orbital radiation. Among these, LPP light sources afford extremely high brightness, close to black body radiation, since they allow increasing plasma density to a substantial degree. Also, selection of the target substance affords light emission in a required wavelength alone. Among other advantages, LPP light sources have moreover a substantially isotropic angular distribution, so that an extremely large capture steric angle, of 2π steradian, can be achieved without structures such as electrodes or the like on the periphery of the light source. Accordingly, such LPP light sources are expected to become prevailing in light sources for EUV lithography in which a power of several tens of watts or greater is required.
The principle of EUV light generation by LPP is explained next. A laser beam is irradiated onto a target substance supplied into a vacuum chamber, to excite thereby the target substance into plasma. Such plasma emits various wavelength components that include EUV light. An EUV collector mirror for selectively reflecting a desired wavelength component (for instance, a component having a 13.5 nm wavelength) is arranged in the vacuum chamber. The EUV collector mirror reflects and condenses EUV light, and outputs the EUV light into the exposure apparatus. As the target substance there may be used, for instance, tin (Sn), lithium (Li), xenon (Xe) or the like. Among these, tin (Sn) is a promising target substance thanks to the high EUV conversion efficiency that it affords. On the reflective surface of the EUV collector mirror there is formed, for instance, a multilayer film obtained by alternately layering a molybdenum (Mo) thin film and a silicon (Si) thin film (Mo/Si multilayer film).
In such an LPP-type EUV light source device, the influence of neutral particles and/or ions emitted from the plasma and target becomes problematic, in particular, when using a solid target. The EUV collector mirror 15 is arranged in the vicinity of the plasma, and hence neutral particles emitted from the plasma and target become adhered to the reflective surface of the EUV collector mirror, impairing the reflectance of the mirror. Meanwhile, the ions emitted by the plasma erode the multilayer film formed on the reflective surface of the EUV collector mirror (this phenomenon is referred to as “sputtering” in the present application). In the present description, contamination refers to the negative effect exerted on optical elements by such neutral particles and ions. Also, the term debris refers to the scattered matter from the plasma, and includes neutral particles and ions, as well as remnants of the target substance.
In order to maintain a high reflectance in the EUV collector mirror, the latter must exhibit high surface flatness, for instance of about 0.2 nm (rms), which is extremely expensive. As a result, frequent replacement of the EUV collector mirror incurs not only longer maintenance downtime but also increased operative costs. Prolonging the life of the EUV collector mirror would be thus desirable in terms of, for instance, reducing the operative costs of the exposure apparatus and of shortening maintenance downtime. The mirror life in an EUV light source device for exposure is defined as the period of time elapsed until a drop of reflectance of, for instance, 10%. Such mirror life must be at least one year.
As described above, a metallic film forms on the surface of the EUV collector mirror through debris adhesion. The metallic film absorbs EUV light, thereby reducing the reflectance of the EUV collector mirror. A hypothetical light transmissivity of the metallic film of about 95% would result in a reflectance of about 90% in the EUV collector mirror. To achieve an EUV collector mirror life of one year or more, the loss of reflectance of the EUV collector mirror for EUV light having a wavelength of 13.5 nm must be no greater than 10%. Consequently, the tolerance for the adhesion amount (thickness) of metal on the reflective surface of the EUV collector mirror is extremely small, of about 5 nm for lithium and of about 0.75 nm for tin.
Debris adheres not only to the EUV collector mirror but also to any components in the chamber. In particular, the output of the EUV light source device increases when debris adheres to the optical elements provided in the optical path of laser beams and/or of the EUV light. Adhesion of debris impairs also the sensitivity of any sensors or the like that may be provided on the optical elements. To prevent such occurrences, thus, the metallic film must be removed periodically from the optical elements in the chamber.
However, the debris particles that form the metallic film have substantial adherence, and hence it has been impossible to fully remove debris from the optical elements, even when subjecting the debris adhered to the optical elements to physical cleaning. Such being the case, US Patent Application Publication No. 2007/0018122 discloses a technology for removing debris from an optical element by subjecting debris adhered to the optical element to a chemical cleaning action and/or a thermal cleaning action.
US Patent Application Publication No. 2007/0018122 discloses a technology of preventing debris from adhering to a chamber window by covering the window with a calcium fluoride and/or magnesium fluoride shield that is itself cleaned. Specifically, the shield is cleaned through chemical etching of the shield surface, using gas and/or plasma, or by causing debris to detach from the shield surface by heating the shield itself to liquefy or vaporize the debris adhered to the shield surface.
As described above, tin (Sn) is a promising target substance thanks to the high EUV conversion efficiency that it affords.
The etching method described in US Patent Application Publication No. 2007/0018122 employs a gas such as hydrogen bromide (HBr), chlorine (Cl2) or the like, or an etchant of the foregoing excited into plasma, for cleaning tin debris. Such a method, however, has the following problems when employed for cleaning an EUV collector mirror. A Mo/Si multilayer film is formed on the reflective surface of the EUV collector mirror. Upon etching of the EUV collector mirror in accordance with the etching method described in US Patent Application Publication No. 2007/0018122, not only the metallic film on the reflective surface, but the reflective surface itself, i.e. the Mo/Si film, becomes etched as well. In addition to impairing as a result the flatness of the EUV collector mirror, this shortens also the life of the EUV collector mirror.
The method for vaporizing debris described in US Patent Application Publication No. 2007/0018122 entails the following problems. To vaporize, for instance, tin debris, the optical element itself must be heated to a temperature of 300° C. or above. The reflective surface and transmission surface of optical elements are covered with various coating substances. Herein, heating the optical element to a temperature of 300° C. or above gives rise to fatigue breakdown on account of differences in the coefficients of thermal expansion of the coating materials, and/or thermal diffusion among coating substances, all of which impair reflectance and/or transmissivity.
Conventional cleaning methods, therefore, have failed to prolong the usable life of EUV collector mirrors by preserving the original reflectance and transmissivity of the optical element over long periods of time.
In light of the above, it is an object of the present invention to solve the problem of extending the usable life of an optical element by making it possible to remove, from the surface of an optical element, debris generated together with EUV light by plasma formed through laser beam excitation of a target in a chamber, without damaging the optical element provided in the chamber.